#include<iostream>
int main()
{
	double n,i=1,k=1,sum=2,s=2,temp,x=1,f;
	scanf("%lf",&n);
	while(i<n)
		{
		temp=s;
		s+=x;
		x=temp;
		sum+=s/x;
		i++;
		}
	printf("%.2lf\n",sum);
	system("pause"); 
	return 0;
}
